Virtually all facilities which use electrical power receive such power from a utility company. Utilities have an excellent record of providing uninterrupted or only-infrequently-interrupted power at proper voltages and line frequency. And when power fails, it usually does so for only a short duration amounting to little more than aggravation. Facility users simply tolerate the occasional disruption of service.
But certain types of facilities, e.g., hospitals and similar critical care institutions, unusual types of manufacturing plants and the like, cannot tolerate a power outage of any significant duration. Those types of facilities are equipped with a standby power system comprising an internal combustion engine driving an electrical generator. When commercial power fails, the engine is automatically started. And when the generator reaches rated voltage and frequency, an automatic transfer switch (ATS) transfers the load imposed by facility equipment from the commercial power lines to the generator. A typical ATS is configured so that its "normal" and "emergency" electrical contacts are electrically operated using solenoid coils. Switches of the foregoing type are disclosed in U.S. Pat. No. 4,157,461 (Wiktor); U.S. Pat. No. 4,423,336 (Iverson et al.); U.S. Pat. No. 4,590,387 (Yoshida et al.) (describing a single transfer switch); U.S. Pat. No. 4,937,403 (Minoura et al.) and U.S. Pat. No. 5,023,469 (Bassett et al.).
If automatic transfer switches (ATS) never failed, it would be the only switch needed to accomplish the foregoing. But for any one of a variety of reasons, failures occur, albeit infrequently. Such reasons include failure or maladjustment of small control-type limit switches, damage to control circuit bridge rectifiers caused by electrical surges, burned-out solenoid coils and even, occasionally, damage caused by disturbances on the commercial power line.
For service-related purposes, a known ATS is configured so that the entirety of the ATS moves on a sliding platform or the like between "operate," "test" and "isolate" positions. The Bassett et al. patent discloses a retractable table for the purpose.
In the first position, the ATS operates to automatically transfer the load to an emergency power source if the normal power source fails. In the second, the ATS control circuit (but not all of the main power-carrying contacts) are connected to the normal power source so that the ATS can be tested or "cycled" through its functions without actually transferring electrical load. In the third position, the ATS is entirely disconnected from the normal power source and can be physically removed for bench service.
The Wiktor patent discloses a mechanism for moving the ATS toward and away from the bypass switch for normal ATS operation or for its testing or removal. The mechanism includes what is often referred to as a bell crank arrangement. The force required to be exerted on the positioning handle varies with the effective lever-arm-length of a link. The Wiktor mechanism is configured so that when moving the ATS from its normal position toward its test position (which requires separation of electrical contacts), the effective length of the link is shortest and highest hand force is required. And assuming the handle moves at a constant angular velocity, the rate of movement of the ATS changes with changes in the effective lever-arm-length of the link.
The ATS withdrawal mechanism disclosed in the Iverson et al. patent is similar to that of the Wiktor patent in that it uses a bell crank arrangement having an effective lever-arm-length which changes as the ATS moves. In the Iverson et al. mechanism, the user rotates a crank which, through a shaft, provides input power to a gearbox. The gearbox output drives a crank arm coupled to a locking bar which, in turn engages a pin on the ATS.
Because automatic transfer switches fail or need occasional service, manufacturers of equipment of this type provide a second, manually-operated bypass switch as a "backup" device. With proper human intervention, the bypass switch also transfers a load between a normal power source and an emergency source. In other words, the bypass switch is essentially redundant in that it performs substantially the same function as the automatic transfer switch. As with an ATS, a bypass switch has a set of "normal" contacts to connect the load to the commercial power line and a set of "emergency" contacts to connect the load to the generator.
Because the automatic transfer switch and the bypass switch both have the described capability of load switching between power sources, great care must be taken to design the switches and their ancillary hardware to assure that the commercial power lines and the generator are not connected to one another. An improperly-designed switch or switch combination could result in such interconnection if (a) the emergency contacts closed before the normal contacts open or vice versa, or if (b) the relative positions of the automatic transfer switch and the bypass switch are not coordinated to prevent such interconnection. (All known manufacturers of individual switches and automatic transfer switch/bypass switch combinations have long since recognized and addressed these concerns.)
A common way to assure that one set of contacts, e.g., the normal contacts, are open before the other set of contacts, e.g., the emergency contacts, close is by using contact "underlap." That is, the switch is constructed in such a way that absent component failure of some sort, simultaneous normal and emergency contact closure is impossible. And a common way to assure that the relative positions of the automatic transfer switch and the bypass switch are properly coordinated to prevent the aforedescribed power line/generator interconnection is by using some sort of interlock arrangement.
Interlock arrangements occur in two broad types, namely, electrical and mechanical. Of these, electrical interlocking is believed to be more common. In general, an electrical interlock system works as follows. Small electrical control switches are in series with the electromagnetic coils energized to close the "normal contactor" (the contactor having the normal contacts thereon) and the emergency contactor. As an example of interlocked operation, the coil for the emergency contactor cannot be energized until the control switch in series with it is closed by the normal contactor having moved to the full open position.
But mechanical interlocks are certainly not unknown. The Bassett et al. patent noted above discloses such an interlock. The interlock system of the Bassett et al. patent might be termed a hybrid in that it has both electromechanical and purely mechanical aspects. The system is configured to prevent the automatic transfer switch and the bypass switch from being simultaneously closed on different, i.e., normal and emergency, sources. Movement of the automatic transfer switch between AUTO, TEST and ISOLATION positions is by another mechanism, only a few components of which are disclosed.
While the known ATS positioning mechanisms have been generally suited for the purpose, they are not without disadvantages. The above-described variable hand force and apparent high hand force are among them. And it appears that the positioning mechanism shown in the Iverson et al. patent acts upon a single point on the ATS being moved. "Cocking" during movement may occur unless the single point is centered on the ATS and the frictional load at the lateral extremities of the ATS are about equal to one another.
Another disadvantage relates to the matter of redundancy. The ATS positioning mechanisms of the Wiktor and Iverson et al. patents use only a single mechanism. There is no redundancy in the event of a failure of such mechanism.
Similarly, certain known mechanical interlock mechanisms are characterized by a good deal of complexity. The interlock system disclosed in the Bassett et al. patent is an example. Electromechanical interlock systems place at least some reliance upon the integrity of electrical coils, small electrical switches and the like. The circuit disclosed in the Bassett et al. patent is an example.
An automatic transfer switch with an improved positioning mechanism and which, when combined with a bypass switch, includes a straightforward yet highly effective interlock mechanism would be a distinct advance in the art.